1. Field of the Invention
The present disclosure relates to a flexible circuit board and a process for producing the same; in particular, to a flexible circuit board formed with a coating-facilitating adhesion enhancing layer on each of the two sides of a compartment having an electric circuit, and a process of producing the flexible circuit board.
2. Description of Related Art
Conventional flexible circuit boards are made by processing precursor substrates. The precursor substrates must be coated with a metal conducting layer to enable subsequent processing. Metals generally do not easily adhere to conventional precursor substrates. Conventional methods of coating metal include metal spraying, sputtering deposition, CVD, vapor deposition, and dry coating. However these methods result in problems of thick precursor substrates, or difficult and overly long coating processes. Excess thickness compromises the miniaturization of products. Difficult and overly long coating processes result in limited production capacity and raised production cost.
The above conventional methods not only cannot overcome problem of necking, but given that the technical and material resources of the precursor substrates are controlled by upstream manufacturers, products that use these precursor substrates are subject to charges by external parties and, due to inability to improve the properties of these products, a fundamental solution for reducing cost is unattainable.
Moreover, in order to form electric circuits on the precursor substrate, an adhesion enhancing metal layer, e.g. nickel, is generally coated on the precursor substrate. Copper is then integrated with the nickel by selectively using a photoresist. Next, large amount of etching is required to remove the copper and nickel that are not part of the electric circuit on the precursor substrate. This method creates large amount of wasted copper and nickel, increasing the cost. In addition to increased cost, the distribution (coverage) of the adhesion enhancing nickel on the substrate is limited, such that improvement upon stable adhesion of the electric circuit is limited, thereby compromising product quality. Portions between the electric circuit and the substrate are not enhanced by the metal adhesion enhancing layer. Different thermal expansion coefficients between the metal and the substrate may create gaps or other structural disintegration in the electric circuit when the ambience temperature changes, thus, affecting the transmission quality and life span of the product. This problem is especially prominent in multi-layered flexible printed circuit boards.
Hence, the present inventor believes the above mentioned disadvantages can be overcome, and through devoted research combined with application of theory, finally proposes the present disclosure which has a reasonable design and effectively improves upon the above mentioned disadvantages.